Diphtheria is a bacterial infection characterized by a thick, gray coating at the back of the throat along with other symptoms mainly affecting mucous membranes in the throat and nose, according to Mayo Clinic. Other symptoms include swollen glands, sore throat, fever and weakness.
Diphtheria is dangerous due to breathing difficulties from the gray coating in the throat. The bacterial infection also poses a risk to the heart, nervous system and kidneys as it progresses due to toxins that spread through the blood, states MedlinePlus. While treatment is available, the infection is still potentially deadly. Routine vaccinations mean diphtheria rarely occurs in developed countries such as the United States.